ARM: 8818/1: dma-mapping: update comment about handling dma_ops when detaching from IOMMU

Update the comment because we don't set the pointer to NULL anymore.
Also use the correct pointer name 'dma_ops' instead of 'dma_map_ops'.

Fixes: 1874619a

 ("ARM: dma-mapping: Set proper DMA ops in arm_iommu_detach_device()")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>